1. Field of the Invention
The present invention relates to a developing device used in image forming apparatuses such as printers, copiers, and facsimile machines that utilize an electrophotography process, an electrostatic recording process, or the like, as well as to an image forming apparatus using such a developing device. In particular, the invention relates to a developing device that develops an electrostatic latent image formed on an image carrying body by using an AC bias as well as to an image forming apparatus using such a developing device.
2. Description of the Related Art
In conventional developing devices that are used, to develop an electrostatic image formed on an image carrying body such as a photoreceptor drum, in image forming apparatuses such as printers, copiers, and facsimile machines that utilize an electrophotography process, an electrostatic recording process, or the like, a technique of applying an AC bias to a developing roller is used widely.
In such developing devices, an oscillating electric field is formed between the developing roller and the photoreceptor drum by applying an AC bias to the developing roller and the oscillating electric field causes developer (hereinafter referred to as "toner") to fly (reciprocate) between the developing roller and the photoreceptor drum, whereby the development efficiency is increased and fogging and the like are prevented.
In image forming apparatuses using such a developing device, there may occur an event that toner sticks to the photoreceptor drum in band-like form when an AC bias being applied to the developing roller is turned off at the end of an electrostatic image development process.
Such sticking of toner (hereinafter referred to as "toner spewing" at the time of turning off of a development bias) occurs in such a manner that in the development region where the developing roller and the photoreceptor drum exist close to each other, when an AC development bias being applied to the developing roller is turned off, van der Waals force, image force, or the like causes toner that is not returned to the developing roller to remain on the surface of the photoreceptor drum in a region where no electrostatic image exists.
In the toner spewing effect at the time of turning off of a development bias, toner particles that are given reciprocative acceleration by the AC electric field force generated by the development bias and that should remain on the developing roller after extinction of the AC electric field force because of their positions, acceleration directions, mass, and charge amounts at the time of the extinction are absorbed on the surface of the photoreceptor drum instead. It is known that because of the above mechanism the amount of spewed toner tends to be influenced by variations in the mass and the charge amounts of toner particles due to moisture absorption and hence is greatly affected by the use environment of the image forming apparatus.
The toner spewing phenomenon not only causes undue consumption of toner but also increases the load on the cleaner. In contact transfer type image forming apparatuses having a transfer roller that directly contacts the photoreceptor drum, bandlike toner remaining on the photoreceptor drum is transferred to the transfer roller and then transferred from the transfer roller to the back surface of a sheet during the next image formation, which means secondary problems such as staining of the back surface of a sheet.
As for techniques capable of preventing the toner spewing out of the developing roller at the time of turning off of a development bias, Japanese Examined Patent Publication No. Hei. 4-35074 discloses a technique in which when a charged surface portion having a uniform potential of the image forming body passes the development region after a latent image portion of the image forming body passed the same region, the bias voltage is changed in such a manner that the maximum voltage of a bias voltage having the same polarity as the uniform potential decreases, whereby a shock at the time of turning off of the bias voltage is reduced and the toner spewing amount is decreased. Japanese Unexamined Patent Publication Nos. Hei. 7-64379 and Hei. 8-202176 propose, with attention paid to the direction of an AC electric field in a jumping development method, an AC-frequency-and-phase-limited development bias turning-off control in which a bias is always turned off in a state that reciprocating toner is moving toward the developing roller. Japanese Unexamined Patent Publication No. Hei. 11-133809 proposes a technique in which during image formation the turning-off itself of a development bias is not performed between pages, whereby the degree of the toner spewing phenomenon is lowered.
However, the above conventional techniques have the following problems. In image forming apparatuses of the AC bias application development type, an AC development bias voltage is applied between the image carrying body and the developer carrying body, whereby an electric field causes plural toner particles existing in the development region where the image carrying body and the developer carrying body are located close to each other to always reciprocate is generated. It is known that constraining force acts on each toner particle in such a manner that its strength depends on the charge amount and the mass of each toner particle.
In particular, in what is called a jumping development method in which the image carrying body and the developer carrying body are separated from each other but located close to each other and an AC development bias is applied, a region (called "toner cloud") where plural toner particles always fly in a reciprocated manner is formed in the space (called "development gap") between the image carrying body and the developer carrying body. The positions, the movement speeds, and the movement directions of the toner particles in the toner cloud region are varied by the time-varying AC electric field. In general, the jumping development method has a problem that it is much more difficult to lower the degree of the toner spewing phenomenon in a stable manner, that is, to establish a state that all toners are returned to the developer carrying body when the development bias is turned off, than in the contact method in which the image carrying body and the developer carrying body are in contact with each other.
In contrast, in the case where, as disclosed in the publication No. Hei. 4-35074, for example, the maximum voltage of the same polarity as the potential of the surface of the image carrying body is gradually decreased, that is, the peak-to-peak voltage of the AC bias is decreased, the movement energies of the toner particles in the toner cloud region decrease and toner particles located close to the surface of the image carrying body cannot return to the developer carrying body and remain on the surface of the image carrying body. If the DC bias is decreased without decreasing the peak-to-peak value of the AC bias, although normally charged toner particles can be attracted to the developer carrying body, what is called "opposite polarity toners," that is, part of the toner particles that are small in charge amount or charged in opposite polarity are positively moved to the image carrying body because the electric field between the image carrying body and the developer carrying body becomes too much stronger than in the developing state (phenomenon called "fogging of opposite polarity"); toner spewing still occurs.
In the AC-frequency-and-phase-limited development bias turning-off control disclosed in the publication Nos. Hei. 7-64379 and Hei. 8-202176 in which the phase of the AC voltage in the jumping development method is controlled so that the bias is always turned off in a state that reciprocating toner particles are moving toward the developing roller, the movement of clouds in which developer is always reciprocating between the image carrying body and the developer carrying body is not such that all developer returns to the developer carrying body in the return phase of one cycle. Therefore, this technique has a problem that developer located close to the image carrying body, toner that is unfavorable in terms of charging (described above), etc. cannot be returned completely and hence the toner spewing cannot be prevented effectively.
The method disclosed in the publication No. Hei. 11-133809 in which the turning-off itself of a development bias is not performed between pages during image formation to avoid toner spewing out of the developer roller at the time of turning-off of the development bias is just a countermeasure for continuous image formation; it is definitely necessary to turn off a development bias at the end of image formation. Therefore, this is not an essential measure.